Powassan virus (POWV) is a flavivirus spread by ticks that causes encephalitis. Cases have been reported in the United States (primarily the northeastern states and Great Lakes region), Canada, and Russia. The vectors responsible for POWV include 4 tick species: Ixodes cookei (American castor bean tick), Ixodes marxi (squirrel tick), Ixodes spinipalpis, and Dermacentor andersoni (Rocky Mountain wood tick). Multiple mammals act as reservoirs for the virus. Transmission of infection usually occurs from June to September.

Asymptomatic infection is thought to be common. The incubation period is 8-34 days. Few patients recall a tick bite. Symptomatic patients present with fever, headache, nonspecific upper respiratory symptoms, gastrointestinal complaints (including vomiting), and weakness. If infection progresses to meningoencephalitis, then confusion, seizures, and hemiplegia may ensue.

Typical cerebrospinal fluid (CSF) analysis findings are similar to those seen in tick-borne encephalitis (TBE): lymphocytic pleocytosis (usually <500 WBC/mm3) and normal to high protein level. MRI of the brain may reveal abnormalities in the parietal and temporal lobes. Significant morbidity with residual neurological impairment occurs in most cases of meningoencephalitis following infection.

Diagnosis is made by measuring CSF or serum for POWV-specific IgM antibodies or detecting a fourfold rise of virus-specific IgG in paired acute and convalescent sera. These tests are not commercially available but can be requested through state health department laboratories and the Centers for Disease Control and Prevention (CDC). The case fatality rate is estimated at 5%-10%. Half of all survivors have permanent neurologic symptoms (recurring headaches, memory problems, and muscle wasting).

Deer tick virus (DTV), also known as Powassan virus lineage II, has also been reported as a cause of encephalitis. It is a Flavivirus antigenically related to POWV, with 84% of its RNA sequence being identical to POWV. The main vector identified to date is Ixodes scapularis. Although the prevalence of adult deer ticks with the virus is high in the north central and northeastern United States, the first case of DTV encephalitis in humans was reported only in 2009.

A 2013 study suggests that some cases of POWV encephalitis reported in patients from the Lower Hudson Valley, NY, between 2004 and 2012 may have in fact been the result of DTV infection.